0346 — Migration registries are explicit ordered slices, not init() registration

Context. The runner needs the full ordered set of system-scope and topic-scope migrations; a common Go pattern is side-effect registration in init() from each migration’s own file. Dated approximately; built across July 2026.

Decision. pkg/system/migrations and pkg/topic/migrations each export an explicit ordered slice, backed by a contiguity unit test — no init() registration.

Consequences. The complete migration sequence is readable in one place and reviewable in a diff; the contiguity test catches a skipped or duplicated version number at test time. Rejected: init() registration — order and completeness become emergent properties of which files got imported rather than a declared list.